<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="vue.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="cityName">
    </div>
    <script>
        var vm = new Vue({
            el:'#app',
            data:{
                cityName: 'shanghai'
            },
            //使用watch监听 cityName变化
            watch:{
                cityName (newName,oldName) {
                    console.log(newName,oldName)
                }
            }
        })
    </script>
</body>
</html>